CAIE IGCSE Chemistry Relative Atomic Mass from Isotopes

Practise calculating relative atomic mass as an abundance-weighted mean, handling percentages or ratios and explaining why the result may be non-integral.

Exam points

multiply each isotope mass by its abundance, sum the products and divide by total abundance
use percentage or ratio data and infer a missing abundance before calculating the weighted mean
explain a non-integral relative atomic mass as the weighted average of naturally occurring isotopes

[Maximum number: 2]

Sulfur is a Group VI element.

A sample of sulfur contains two isotopes, ${ }^{32} \mathrm{~S}$ and ${ }^{34} \mathrm{~S}$ .

Table 5.2 shows the relative abundance of these isotopes of sulfur in the sample.

Table 5.2

Calculate the relative atomic mass of sulfur in this sample to one decimal place.
relative atomic mass =

$\mathrm{M} 1(32 \times 95)+(34 \times 5)=3210$

M2 3210 / 100=32.1

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2.3.4—Relative atomic mass of an element question 1